1. Field of the Invention
The present invention generally relates to charge transfer apparatus and, more particularly, to a charge transfer apparatus having a so-called floating diffusion amplifier that converts a signal charge from a charge transfer section formed of a CCD (charge-coupled device) into an output voltage.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows a structure of a main portion (output section) of a CCD solid state imager device according to the prior art. As shown in FIG. 1, the conventional CCD solid state imager device, particularly, its output section comprises a charge transfer section 21 formed of a CCD, a discharge element 22 composed of a floating diffusion FD, a reset gate RG and a drain region (hereinafter referred to as a reset drain, if necessary) RD connected to the next stage of the charge transfer section 21 through an output gate OG and an output amplifier 23 formed of an output element Q1 and a load resistance element Q2 connected to the next stage of the discharge element 22.
A signal charge transferred from below a transfer gate electrode TG of the final stage of the charge transfer section 21 is temporarily accumulated in the floating diffusion FD. Then, the voltage change based on the accumulated electric charges is supplied to the output amplifier 23 of the succeeding stage, thereby being output from an output terminal .phi.out of the output amplifier 23 as an output voltage S.
After the output voltage S was generated from the output terminal .phi.out of the output amplifier 23, a reset pulse Pr is supplied to the reset gate RG, whereby the floating diffusion FD is reset to a drain voltage Vdd and electric charges accumulated in the floating diffusion FD are discharged to the reset drain RD side.
In the conventional CCD solid state imager device, the reset gate RG is formed of a MOSFET (metal oxide semiconductor field effect transistor) of a depletion type having a channel region 24 whose carrier concentration (donor impurity concentration) exceeds 5.times.10.sup.16 cm.sup.-3. As a consequence, an amount that a potential formed under the reset gate RG is fluctuated when an amount of implanted donor impurities and the drain voltage Vdd supplied to the reset drain RD are fluctuated is considerably increased. Therefore, a driving range of the reset gate RG must be widened by increasing a margin of an amplitude of the reset pulse Pr supplied to the reset gate RG or a low level of the reset pulse Pr must be designated at every device.
When the reset gate RG is formed of a MOSFET of an enhancement type, the donor impurity need not be implanted into the channel region 24. Therefore, an amount that the potential formed under the reset gate RG is fluctuated is reduced (in this case, the potential is fluctuated only by the fluctuation of the drain voltage Vdd). However, the high level of the reset pulse Pr exceeds the level of the drain voltage Vdd so that electric charges accumulated in the floating diffusion FD and electric charges accumulated in the drain region RD are implanted under the reset gate RG. There is then the disadvantage such that the charge transfer apparatus is degraded in reliability.